1. Field of the Invention
This invention relates to rare earth phosphors. More particularly, the invention relates to a method for preparing such phosphors (generally in association with a rare earth activator) which involves a treatment with a silicon-containing sensitizer material, and to the resultant rare earth phosphors (and associated activator) incorporating retained sensitizer and, optimally, fluorine. The term "rare earths" as used in the present specification refers to yttrium and scandium plus the metals in Group III of the Periodic Table generally classified as lanthanide rare earths, to wit: lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. The term "phosphor" refers to a material which is capable of exhibiting luminescence when subjected to appropriate excitation. The term "rare earth activator" refers to compounds of rare earth elements which may be combined with other rare earth compounds to activate luminescence thereof, including, for example, compounds of europium, terbium, erbium, thulium, dysprosium, ytterbium and prasodymium.
2. Description of the Prior Art
Luminescent properties of certain rare earth-containing compositions have long been recognized. In recent years, rare earth phosphors have received considerable attention and have been the subject of many intensive investigations. The recent interest is due partly to the discovery that certain rare earth phosphors, particularly the oxygen-bearing phosphors (oxyphosphors) can be used advantageously as cathodoluminescent coatings for color television tubes. In general, the rare earth phosphors are in the form of a solid solution having a matrix of rare earth compounds such as a rare earth oxide or vanadate and an activator which is commonly called a "dopant" and generally is also a rare earth element.
The effectiveness of the activator is dependent to a large extent on its intimate relation within the rare earth matrix. To insure the formation of an intimate mixture, rare earth phosphor manufactures may prefer to dissolve into an acid solution the rare earth element in the form of an oxide, together with the activator, to form a homogeneous solution. The rare earth element and the activator are then coprecipitated from solution in the form of oxalate, hydroxide, carbonate or sulfate. The precipitate may be recovered and fired at a high temperature to decompose the salts into mixed oxides in powder form. This finely divided, reactive form is favorable for reaction with certain oxy-acids to synthesize such oxygen-dominated and europium-activated phosphors as yttrium vanadate, gadolinium vanadate, yttrium tungstate, yttrium germanate, gadolinium aluminate, etc., and conditions can be adjusted to yield desirable crystal growth and particle size distribution. The phosphors thus synthesized may be used as luminescent coatings for color television tubes and other applications.
Prior art methods reported for preparing various rare earth activated oxysulfides and prior art disclosures of various rare earth activated oxysulfide phosphors are to be found in U.S. Pat. Nos. 2,402,547; 3,418,247; 3,418,246; 3,423,621; 3,502,590; 3,515,675; 3,562,174; and 3,563,909. Many of these methods involve the reaction of a rare earth compound with a sulfur-containing gas at an elevated temperature to form the oxysulfide. Still another technique reported, only for the preparation of europium activated lanthanum, gadolinium, yttrium and lutetium oxysulfides, is the solid state reaction between the mixed rare earth oxides (activator and host) and a composition such as alkali metal carbonates and sulfur which produces alkali metal sulfides and polysulfides, which in turn react at elevated temperatures with the rare earth oxides to form the rare earth oxysulfides. In addition to the alkali metal carbonate, it is reported that an alkali metal sulfate, phosphate, arsenate, or germanate may be used.
U.S. Pat. No. 3,415,757 discloses fluoro-substituted europium activated gadolinium, yttrium and lanthanum oxide phosphors (oxyfluorides) having a fluorine content lying between 0.1 and 5.0% by weight. These products are made by heating a mixture of rare earth oxides with ammonium fluoride, or preferably europium fluoride.
Although the phosphors described above are being employed as luminescent coatings for color television tubes and in other applications, these phosphors do not generally have the combined properties of being free flowing, having high brightness and controlled particle size. The methods described above do not suffice for controlling the crystal growth properly, and generally lead to wide particle size distribution or result in crystal growth at the expense of lowering the phosphors optical properties.